1. Technical Field
The present invention relates to a circulation member or part for circulating balls of a ball screw and also relates to a ball screw using the circulation member.
2. Related Art
A ball screw generally comprises a screw shaft having an outer peripheral surface on which a ball rolling groove having a spiral or helical shape is formed, a nut member having an inner peripheral surface on which a ball rolling groove having a spiral or helical shape is formed, and a number of balls disposed between a loaded ball rolling passage formed by the ball rolling grooves of the screw shaft and nut member. In this structure, when the screw shaft is rotated relatively to the nut member, the balls roll in the loaded rolling passage formed by the screw shaft and the nut member.
FIG. 9 shows one example of a nut member, to which a return pipe 3 as a circulation member is mounted so as to connect one and the other end portions of a loaded rolling passage 2. The return pipe 3 has a portal shape having a leg portion of which end portions are provided with scooping portions for scooping the balls. The return pipe 3 acts to scoop, at its one end, the balls rolling on the loaded rolling passage 2, to guide them inside the return pipe 3 and then to return them to the loaded rolling passage from the other end thereof. According to such action, the balls rolling on the loaded rolling passage 2 circulate, in the endless manner, in the circulation passage composed of the loaded rolling passage 2 and the return pipe 3.
Recently, in order to ensure the smooth circulation of the balls, that is, to smoothly guide the balls rolling on the loaded rolling passage 2 into the return pipe 3 and smoothly return the balls moving in the return pipe 3 to the loaded rolling passage 2, there has been adopted a structure for scooping the balls to the loaded rolling passage 2 from a direction tangential thereto. In such structure, a pair of leg portions 3b, 3b disposed at both sides of the central portion of the return pipe 3 are tilted (inclined) by a predetermined angle each in different direction with respect to the central portion 3a so as to correspond to a lead angle of the screw shaft.
According to a known method of such return pipe 3, an iron tube is bent so as to provide a portal shape and a scooping portion is then machine-worked. In this known method, however, much manufacturing cost is involved and it is difficult to bend the tube with predetermined dimensional performance, providing a problem. Especially, when it is required to incline the leg portions 3b, 3b of the return pipe 3 in the directions different from each other in accordance with the lead angle of the screw shaft, it is necessary to bend the tube three-dimensionally, which results in further difficulty in working.
FIG. 10 shows another example of a conventional return pipe 4. With reference to FIG. 10, the return pipe 4 is divided into two parts 4a, 4a in a plane including an axial line of the pipe. These two parts 4a, 4a are produced by press-working a metal plate member by using a mold so as to provide a semi-circular cross section.
In such return pipe manufacturing method through the press-working, in a case where the leg portions 5, 5 of the return pipe 4 are bent in one plane, i. e., where the leg portions 5, 5 are not inclined in correspondence to the lead angle of the screw shaft, joining (or joined) surfaces 6, 6 of the respective divided parts 4a, 4a are positioned in the same one plane, so that the respective divided two parts 4a, 4a can be easily manufactured. However, on the other hand, in a case where the leg portions 5, 5 are inclined in different directions from each other in accordance with the lead angle of the screw shaft, the joining surfaces 6, 6 of the respective divided parts 4a, 4a are not positioned in the same one plane and provide such a shape as like as twisted. Therefore, it is extremely difficult to manufacture the return pipe 4 by the press-working of the metal plate member.